This invention relates to the sampling of environmental air for the purpose of measuring levels of airborne contaminants for protection against pollutant related diseases. For a number of years personal and area sampler pumps have been used to draw air samples of known volumes through collection devices, such as filters, to collect particulates in the sampled air volume, and sorbent tubes to trap vapors and gases for future analysis, as well as direct reading colorimetric indicator tubes. Pumps have also been used for direct collection of air samples for analysis. Although fixed volume grab samples are sometimes taken, these are usually for reasons of immediate safety, and for long term health protection, the air sampled should be taken at a constant rate over an extended period of time to provide a time-weighted average measure of the contaminant concentration. Personal sampler pumps are designed to be worn by the individual being monitored for a number of hours, so as to obtain a measure of the average concentration of contaminant breathed by an ambulatory worker or other individual at various locations. Area sampler pumps are usually larger and stationary, and they are designed to sample much larger volumes of air at higher flow rates, to determine average contaminant concentration over time at a particular location.
The health hazard caused by airborne asbestos fibers has become widely recognized in the past decade. Various governmental regulations on the federal, state and local levels have been promulgated for the removal of asbestos from existing structures and vehicles. Asbestos had been so widely used for insulation and fire protection that complete removal from schools and other public and private buildings, as well as ships, will require several more years. Immediate application of the subject invention is for area monitoring at sites of asbestos removal, both to determine the existing hazard posed by prior levels of airborne asbestos, as well as to assure the safety of an area after completion of asbestos removal. The application is not limited to asbestos monitoring, however, as there are continuing hazards from other airborne dusts such as silica, cotton dust, etc., which provide requirements for an improved sampler. The invention also has application as an improved personal sampler, both for particulates and gas monitoring.
There are certain limitations of sampler pumps currently available. Existing portable pumps do not use direct measurement of the true volumetric flow rate to set and control the flow rate of sampled air, but these are usually inferred. The flow rate is frequently set by connecting the pump to an external meter at the beginning of the sampling period, and an inferential control is used to maintain constant flow. Also, where a flow indicator is supplied with the pump, it is usually of poor accuracy, such as a small rotameter, and it is located on the outlet of the pump where an erroneous indication can occur due to leakage in the pump and pneumatic line. This is illustrated by current regulations for asbestos sampling, in which the outlet flow rate must be measured with the inlet blocked to test for leaks. There is, at present, no device that uses selection and direct measure for control of the flow of primary interest between the collection device and the pump in units of air volume at ambient conditions. Since the pressure and temperature in the line between the collection device and pump can vary, it is necessary to correct the direct measurement of this flow rate to provide the actual atmospheric volume being sampled, upon which standards are based.
Baker and Clark in U.S. Pat. No. 4,063,824 show a control in which the pressure drop across a constant orifice (or valve) is maintained at a constant value by means of a pressure switch and integrator, which vary the pump speed. To change the flow rate, however, an external flowmeter must be connected, and the valve setting changed, a procedure which is difficult to accomplish satisfactorily in the field.
Lalin in U.S. Pat. No. 4,432,248, and Hollenbeck in U.S. Pat. No. 4,237,451 describe control systems in which the flow rate is manually set prior to sampling, and the flow rate is controlled by adjusting pump speed in relation to increase in motor current caused by loading of a (particulate) collection filter.
Some area sampling pumps are driven through line cords by AC motors, whose speeds cannot be readily changed. In an attempt to maintain constant flow rate during a test, a critical orifice is sometimes placed between the collection filter and the pump. The large pressure drop across the orifice to maintain critical flow is wasteful of energy, and variations in the high pressure drops across the collection filter associated, for example, with asbestos sampling, can introduce considerable error.
At asbestos removal sites, area samplers that use AC motors are frequently stopped or interrupted by power failures or disconnection of the line cord, which can stop or destroy the validity of a test.
Total sampled air volume is a primary measured quantity, which must be known to calculate concentration. Present pumps use timers which infer the volume from an assumed constant flow rate. Even when electronic timers are included as part of the pump control, the reading can be lost due to power interruption.
There is a need for a controlled sampler in which a true volumetric flow rate can be variably selected as part of the control system, and in which the true volumetric flow rate through the collection device is directly measured and controlled.
Ther is also a need for a controlled sampler in which the total volume is measured and retained in a memory, even in the event of power failure, until physically reset.
There is a further need for a controlled area sampler that can be powered from an AC line or battery, and in which an auxiliary battery can be connected so that a smooth transfer from the line to the battery occurs in the event of a power failure, preserving the continuity of a test.
It is the purpose of the present invention to meet these needs, as well as other objectives.